A fixed multi-beam antenna system is one of several possible implementations in telecommunication systems. The principle is to have a fairly large set of fixed beams where each beam covers a part of the service area for a node and all beams together covers the complete desired service area with two orthogonal polarizations.
In such systems downlink and uplink transmission between a UE and the node can be scheduled by making use of a subset of beams that are the most appropriate, i.e. the beams that gives the best link budget and has the best SIR. At the same time it is also possible to schedule other UE's by making use of another sub-set of beams assuming that the interference can be kept sufficiently low i.e. that the spatial isolation between the different subsets of beams is sufficient.
One way to realize a multi-beam antenna system is to make use of array antennas and having each beam corresponding to a set of element excitations (element weights).
One issue with a fixed multi-beam system, as in all systems, is that a signal will be transmitted and received not only via a desired beam but also via side lobes, i.e. the spatial isolation between beams is dependent on the side lobe levels of the beams. The signal transmitted and received via side lobes will appear as interference to other signals transmitted via other beams and will make simultaneous transmission to and/or reception from several users (MU-MIMO), or a single user (SU-MIMO), less favorable.
Thus good isolation between the beams requires that each beam has not only the desired coverage area but also low side lobe levels outside the coverage area in order to give low interference to the other beams.
When using array antennas, the element weights for each beam should be determined for desired beam coverage as well as for low side lobe levels. However, in general the requirement of low side levels means a penalty in reduced aperture efficiency. In down link also a poor PA utilization will be an issue in the case of an active array antenna realization.